Design and analysis of high electron mobility transistor inspired: III-V electro-optic modulator topologies

Abstract

III-V heterostructure based high electron mobility transistors (HEMTs) offer superior performance as compared to CMOS silicon transistors owing to the high mobility in the 2D electron gas (2DEG) channel at the heterostructure interface. Gallium nitride (GaN) based HEMTs are also suitable for high power and high temperature applications. GaN has a rich offering of material properties spanning domains of non-linear optics, piezoelectric micro-electro-mechanical systems, and monolithic microwave integrated circuits. In this paper, we propose HEMT inspired III-V electro-optic modulator topologies and study the light interaction with 2DEG in such structures. We analyze the electroabsorption and electrorefraction on account of the 2DEG interaction with light and present a design framework to selectively leverage the desired mechanism of modulation. Our analysis suggests that modulation index of electrorefractive modulation in a HEMT-like structure is comparable to silicon photonic modulators. By leveraging the higher electron mobility in these material platforms, the device concepts proposed here could potentially be used to design electro-optic modulators with extremely high modulation rates.

Design and analysis of electro-optic modulators based on high contrast gratings in AlGaN/GaN heterostructures

“…The AlGaN layer is the top layer in the HCG bar, with thickness 25 nm and refractive index of 2.15 at 1.55 µm wavelength. In our previous work [22] we have analyzed electro-optic modulation due to plasma dispersion effect of 2DEG in an AlGaN/GaN HEMT, wherein we observed that the electron wavefunctions in the 2DEG are highly localized within a distance of 10 nm at the AlGaN-GaN interface, which is also consistent with other reported calculations [42]. Therefore the thickness of the 2DEG layer in the multi-layer HCG is considered as 10 nm.…”

“…Even though the 2DEG layer is highly localized at the AlGaN-GaN interface, and has calculated spatial extent of merely 10 nm with small change in refractive index (3.5 × 10 −3 ) due to applied gate voltage, incorporating the HEMT into an HCG structure results in large change in reflectivity (>50 %) by switching the 2DEG from ON (saturation) to OFF (depletion) state. This boost in modulation efficiency is significant improvement from HEMT based modulator architectures presented in our previous work [22]. The HCG resonance wavelength showing a linear dependence on the GaN thickness and duty cycle (η) of the gratings.…”

“…GaN being a wide bandgap material also finds applications in the visibleultraviolet spectral range and THz devices [31][32][33]. Being an atomistically thin material, graphene suffers from limited spatial-interaction with light limiting modulation depth to 2.3 % [29,34] similar to 2DEG EOM proposed in our earlier work [22]. Graphene processing is CMOS compatible, and some recent studies show adoption of GaN-on-silicon platform [35,36] enabling CMOS compatibility.…”

“…Therefore the thickness of the 2DEG layer in the multi-layer HCG is considered as 10 nm. We have also calculated electro-refraction induced change in refractive index of magnitude 3.5 × 10 −3 for change in gate voltage ∆V g ≈ 6 V [22]. Therefore the refractive index of the 2DEG layer is set to be the same value as GaN (2.31) in OFF state (i.e when the 2DEG is not present) and 2.3065 in ON state (i.e when 2DEG is present).…”

“…In this paper we extend the HCG resonator concept to explore the design space for realizing 2DEG based modulators, wherein voltage-mediated modulation of the 2DEG concentration is utilized to modulate the reflectivity of the HCG resonator mirror. We develop an analytical model for multi-layer HCG structure, wherein the 2DEG layer is modeled as a thin layer sandwiched between the AlGaN and GaN films, whose refractive index is computed using Drude plasma dispersion model presented in our previous work [22]. We study the feasibility of realizing such modulators in AlGaN/GaN HEMTs analyzed as multi-layer HCG structures, and present geometries that produce a change in reflectivity as large as 70 % for C-and L-band wavelengths.…”